Christine Quinn walks along the route of the Douglaston Memorial Day parade on Monday .

New York City mayoral candidate Christine Quinn’s lead continued to slide in the battle for the Democratic nomination following former U.S. Rep. Anthony Weiner’s entry into the race, but hypothetical matchups in a probable runoff election showed she would defeat each of her top rivals, a poll released Tuesday showed.

The Marist College poll, taken after Mr. Weiner announced his candidacy by online video on Wednesday, showed Ms. Quinn, the City Council’s speaker, at 24% and Mr. Weiner, who resigned from congress two years ago following a sexting scandal, in striking distance at 19%. Ms. Quinn’s standing in the poll, which includes voters who are undecided yet leaning toward a candidate, is her lowest yet in the hotly competitive race.

The other contenders for the Democratic nomination trailed, with Public Advocate Bill de Blasio, at 12%, former city Comptroller Bill Thompson, 11%; Comptroller John Liu, 8%; and former Council Member Sal Albanese, 1%. Among registered Democrats, 23% said they were undecided.

For the first time this election season, Marist examined how Ms. Quinn would fare in a potential runoff, and the results show she would beat each of her main contenders. If a candidate fails to win 40% of the vote in the Sept. 10 primary, a runoff is held between the top two vote getters, and a spate of polls suggests there’s little chance any candidate in the crowded field could win the primary outright.

“The dynamic of the race clearly shifted with Weiner getting into it,” said Lee Miringoff, director of the Marist Institute for Public Opinion. “Weiner is very clearly a factor in all this and can’t be overlooked.”

On Tuesday afternoon, Mr. Weiner will appear on stage for the first time with most of his main contenders at an education forum. Ms. Quinn is the only major Democrat who will not be at Tuesday’s forum.

Ms. Quinn’s standing in the polls has plummeted during the last few months. She scored a high of 37% in February, a survey that didn’t include Mr. Weiner as a possible candidate. In a poll last month, when Mr. Weiner was included in the survey, Ms. Quinn attracted support from 26% of voters, compared with 15% for Mr. Weiner.

“Christine Quinn is focused on laying out her ideas for New York and making the case that she is the only candidate in the race that has a record of getting things done for middle class New Yorkers,” said Quinn campaign spokesman, Mike Morey.

Among Democrats, 39% said they are strongly committed to their choice and another 35% said they are somewhat committed, while 25% said they might vote differently. Of those who support Ms. Quinn, 30% are strongly behind her and 42% somewhat support her, while nearly a quarter said they might vote differently. Mr. Weiner’s supporters are more committed than the speaker’s, with 43% strongly behind him and another 38% somewhat supportive; among Mr. Weiner’s supporters, 17% said they might vote differently.

Ms. Quinn, who lives in Manhattan, outperformed her rivals in every borough except Brooklyn, where Mr. Weiner beat her 21% to 14%. In Congress, Mr. Weiner represented a swath of Brooklyn and Queens; he currently lives with his wife and son in Manhattan.

Among every racial group, Ms. Quinn accrues more voters than each of her rivals. Among Jewish voters, Mr. Weiner beats the speaker, 23% to 17%. Mr. Weiner had a slight edge over Ms. Quinn, albeit not statistically significant, among men. Ms. Quinn, who aspires to be the city’s first female and firstly openly gay mayor, leads among women.

The rankings among the top contenders remained unchanged when Marist looked at likely voters. The poll showed that among those Democrats who are likely to vote in the primary, 24% support Ms. Quinn, followed by 19% for Mr. Weiner, 14% for Mr. de Blasio, 13% for Mr. Thompson and 8% for Mr. Liu.

Two years ago, Mr. Weiner resigned from congress after he admitted that he sent sexually explicit photos of himself to women on Twitter and then lied about it. A majority of registered voters, 53%, said Mr. Weiner deserves a second chance, while 39% said they don’t believe Mr. Weiner has the character to be mayor. Among Democrats, 59% said they think he deserves a second chance. But 61% of Republican voters said he doesn’t have the character to be the city’s chief executive.

Democrats were overall divided in their impressions of Mr. Weiner, with 44% saying they view him favorably and 44% who said they did not. Ms. Quinn is viewed favorably by 60% of registered Democrats and unfavorably by about a quarter.

Interest in the mayor’s race, thus far, remains relatively low. Among registered voters, 41% said they are paying attention to the campaign, while 43% said they are not following the race very closely, and another 16% said they are not paying attention at all.

A majority of city voters, 52%, said they think the city is moving in the right direction, and 37% said they believe it is going in the wrong direction. Approval ratings for Mayor Michael Bloomberg, who is slated to step down in December after 12 years in office, remained steady, with 48% saying he’s doing an excellent or good job.

The survey of 1,001 city adults, including 810 registered voters and 492 Democrats, was taken Wednesday through Friday of last week. The margin of error for results with registered voters is plus or minus 3.4 percentage points; the margin of error for Democrats is plus or minus 4.4 percentage points.